This invention relates to tool inserts and more particularly to cutting tool inserts for use in drilling and coring holes in subterranean formations.
A commonly used cutting tool insert for drill bits is one which comprises a layer of polycrystalline diamond (PCD) bonded to a cemented carbide substrate. The layer of PCD presents a working face and a cutting edge around a portion of the periphery of the working surface.
Polycrystalline diamond, also known as a diamond abrasive compact, comprises a mass of diamond particles containing a substantial amount of direct diamond-to-diamond bonding. Polycrystalline diamond will generally have a second phase which contains a diamond catalyst/solvent such as cobalt, nickel, iron or an alloy containing one or more such metals.
In drilling operations, such a cutting tool insert is subjected to heavy loads and high temperatures at various stages of its life. In the early stages of drilling, when the sharp cutting edge of the insert contacts the subterranean formation, the cutting tool is subjected to large contact pressures. This results in the possibility of a number of fracture processes such as fatigue cracking being initiated.
As the cutting edge of the insert wears, the contact pressure decreases and is generally too low to cause high energy failures. However, this pressure can still propagate cracks initiated under high contact pressures; and can eventually result in spalling-type failures.
In the drilling industry, PCD cutter performance is determined by a cutter's ability to both achieve high penetration rates in increasingly demanding environments, and still retain a good condition post-drilling (hence enabling re-use). In any drilling application, cutters may wear through a combination of smooth, abrasive type wear and spalling/chipping type wear. Whilst a smooth, abrasive wear mode is desirable because it delivers maximum benefit from the highly wear-resistant PCD material, spalling or chipping type wear is unfavourable. Even fairly minimal fracture damage of this type can have a deleterious effect on both cutting life and performance.
With spalling-type wear, cutting efficiency can be rapidly reduced as the rate of penetration of the drill bit into the formation is slowed. Once chipping begins, the amount of damage to the table continually increases, as a result of increased normal force now required to achieve a given depth of cut. Therefore, as cutter damage occurs and the rate of penetration of the drill bit decreases, the response of increasing weight on bit can quickly lead to further degradation and ultimately catastrophic failure of the chipped cutting element.
In optimising PCD cutter performance increasing wear resistance (in order to achieve better cutter life) is typically achieved by manipulating variables such as average diamond grain size, overall catalyst/solvent content, diamond density and the like. Typically, however, as PCD material is made more wear resistant it becomes more brittle or prone to fracture. PCD elements designed for improved wear performance will therefore tend to have poor impact strength or reduced resistance to spalling. This trade-off between the properties of impact resistance and wear resistance makes designing optimised PCD structures, particularly for demanding applications, inherently self-limiting.
If the chipping behaviours of more wear resistant PCD can be eliminated or controlled, then the potentially improved performance of these types of a PCD cutters can be more fully realised.
Previously, modification of the cutting edge geometry by bevelling was perceived to be a promising approach to reducing this chipping behaviour. It has been shown (U.S. Pat. Nos. 5,437,343 and U.S. Pat. No. 5,016,718) that pre-bevelling or rounding the cutting edge of the PCD table significantly reduces the spalling tendency of the diamond cutting table. This rounding, by increasing the contact area, reduces the effect of the initial high stresses generated during loading when the insert contacts the earthen formation. However, this chamfered edge wears away during use of the PCD cutter and eventually a point is reached where no bevel remains. At this point, the resistance of the cutting edge to spalling-type wear will be reduced to that of the unprotected/unbevelled PCD material.
U.S. Pat. No. 5,135,061 suggests that spalling-type behaviour can also be controlled by manufacturing the cutter with the cutting face formed of a layer of PCD material which is less wear resistant than the underlying PCD material(s), hence reducing its tendency to spall. The greater wear of the less wear resistant layer in the region of the cutting edge provides a rounded edge to the cutting element where it engages the formation. The rounding of the cutting edge achieved by this invention hence has a similar anti-spalling effect to bevelling. The advantages of this approach can be significantly outweighed by the technical difficulty of achieving a satisfactorily thin, less wear resistant layer in situ during the synthesis process. (The consistent and controlled behaviour of this anti-spalling layer is obviously highly dependant on the resultant geometry). In addition, the reduced wear resistance of this upper layer can begin to compromise the overall wear resistance of the cutter—resulting in a more rapid bluntening of the cutting edge and sub-optimal performance.
JP 59-219500 claims an improvement in the performance of PCD sintered materials after a chemical treatment of the working surface. This treatment dissolves and removes the catalyst/solvent matrix in an area immediately adjacent to the working surface. The invention is claimed to increase the thermal resistance of the PCD material in the region where the matrix has been removed without compromising the strength of the sintered diamond.
A PCD cutting element has recently been introduced on to the market which is said to have improved wear resistance without loss of impact strength. United States patents U.S. Pat. Nos. 6,544,308 and 6,562,462 describe the manufacture and behaviour of such cutters. The PCD cutting element is characterised inter alia by a region adjacent the cutting surface which is substantially free of catalyzing material. The improvement of performance of these cutters is ascribed to an increase in the wear resistance of the PCD in this area; where the removal of the catalyst material results in decreased thermal degradation of the PCD in the application.
Whilst removing the catalyst/solvent in this region substantially reduces the incidence of the highly detrimental spalling failure on the leading edge, spalling-type failure on the trailing edge, which originates from characteristic lamellar-type cracking in this region, can also have a significant effect on performance. Although the stresses in the region of the trailing edge are not as high as those on the leading edge, cracking in this area can cause substantial material loss and hence degrade the performance of the cutter.